GaN-based high electron mobility transistors (HEMTs) are high-frequency transistors suitable for applications in communication satellites, power amplifiers and other high-power electronic devices. Growth of GaN on a bulk Si platform offers a cost effective method that may lead to high manufacturing yield compared to conventional methods using sapphire and SiC substrates. However, high lattice and thermal mismatch between Si and GaN leads to tensile stress which results in cracking, high density of threading dislocations and severe epilayer bowing due to in-plane strain. The non-uniform composition/thickness in the layer structure complicates the electrical properties and leads to compromised device performance. Further, the inability to grow thick, crack-free GaN layers make them unsuitable for application in high voltage devices. The severe bowing of the substrate also disallows the use of thin bulk Si as a substrate. Due to these reasons, large scale commercial application of high-voltage Ga-on-Si devices has been difficult, consequently hampering the development of high performance, cost-effective HEMT devices which could have significant industrial implications.
Several techniques have been developed in an attempt to relieve the stress in the layer structure and to create crack-free GaN-on-Si. This includes the incorporation of superlattices or low-temperature AlN interlayers in the GaN layer. Although these methods are useful for LED applications, in the case of HEMT they give rise to multilayers of two-dimensional electron gas (2DEG) interfaces which cause the device to become leaky and less efficient as line defect density increases. Further, the incorporation of these features cause the growth time of the layers to increase, limiting the time and cost efficiency of the production of the device.
There is therefore a need to provide a high-performance, cost-effective GaN-on-Si device that overcomes, or at least ameliorates, one or more of the disadvantages described above.